The invention relates to combination electric and microwave ovens generally and more particularly to an air flow system to enhance the application of thermal energy or microwave energy or both forms of energy in a common oven cavity.
Combination ovens combining the capability of microwave energy with the capability of radiant energy or circulated hot air are well known in the art. Numerous forms of these ovens have appeared on the market and are available to the consumer at the retail level. In addition, several United States patents have issued disclosing various aspects of these ovens. For example, U.S. Pat. No. 3,514,576 discloses a combination oven that simultaneously exposes food to microwave energy and heated air. U.S. Pat. No. 3,569,656 discloses a control concept for controlling the application of infrared radiation and microwave radiation in an oven. U.S. Pat. No. 4,129,769 discloses a timer arrangement for a combination oven. U.S. Pat. Nos. 4,028,520, 4,096,369, and 4,097,709 each disclose air flow systems for such ovens.
It is generally accepted that circulating the air heated by an electric resistance heater will ovenbrown food faster than air naturally convected by differences in temperature caused by the heater. The improved heat transfer associated with forced convection ovens results in savings in time and energy. Additional savings are realized where the heat is evenly distributed.
Even heat distribution and improved heat transfer permit the circulated hot air or convection oven user to reduce the oven temperature without unduly lengthening the cooking time. In fact, most foods cook at lower temperatures and for less time than in a predominately radiant-heat-transfer oven. This, in turn, keeps the oven walls cleaner by reducing the likelihood of grease splatter from the cooking food. And, since the food surface is cooked faster, the food is more moist and more flavorful because the natural juices and flavors are sealed inside very quickly.
One difficulty with prior attempts to combine cooking with microwaves with electric resistance heat, whether circulated or not, has been the inability to do so using normal household voltage and current. This is primarily a result of the power requirements normally associated with portable microwave and electric ovens that operate from 110 to 120 volts. Both are generally designed to draw approximately 15 amperes of current. To draw much less than 15 amperes would substantially impair the smaller ovens' ability to cook quickly. To combine them and apply them simultaneously in one cooking operation, then, would require approximately 30 amperes of current. This is not typically or normally available from a household outlet.
One solution employed in the past has been to manually apply microwave energy and radiant energy sequentially to the food. Numerous microwave ovens have appeared on the market equipped with "browning" elements or sheathed resistive elements to brown food. U.S. Pat. No. 4,129,769, referred to earlier, discloses a timer arrangement for such a device. The difficulty with manually switching the two forms of energy is that they are not truly combined in one cooking operation. In the microwave mode, the browning element does not absorb energy to get hot enough to contribute significantly to the cooking process. In the electric mode, the microwave generator or magnetron does not function at all. Hence, the cooking process is confined to two steps, cooking with microwaves followed by thermal browning or vice versa.
Another possible solution would be to cut the power requirements of the microwave oven and the electric oven to limit their total current requirements to approximately 15 amperes when they are operated simultaneously. To do so, though, would greatly increase the time required to cook with microwave energy alone. This would, of course, negate the single most important reason for cooking with microwaves-speed.
Hence, commercial products incorporating truly dual microwave and electric cooking capabilities have been limited to specially provided, single location 220-240 volt sources when cooking with both modes simultaneously has been desired. This is the same power source used by traditional free-standing and built-in ranges, and the truly dual mode ovens have been only so built as distinguished from countertop or portable ranges or ovens that operate off ordinary wall outlets.
In co-pending U.S. Patent Application Ser. No. 049,454, now abandoned hereby incorporated by reference, filed June 18, 1979, in the names of Buddy J. Austin and John R. Copping and assigned to the same assignee as the present invention, there is disclosed an electrical circuit for a truly dual microwave and electric oven that uses normal household voltage and current to cook by dielectric heating from microwaves and thermal heating from an electric resistance heater. As disclosed in the Austin et al. application, circulation of the air heated by an electric heater past the food further reduces the time and energy required to cook the food. By carefully controlling and utilizing that air and the air used to cool the electrical components of the oven, yet further savings are realized, and an even superior food products results.